Syndiotacticity and isotacticity involve two broad classes of stereospecific structure formations which may be involved in the formation of stereoregular polymers from various monomer units. Syndiotactic polymers, such as syndiotactic polypropylene, have a stereochemical structure in which the monomeric units have an enantiomorphic configuration in which the methyl groups on the asymmetrical carbon atoms follow each other alternatively and regularly in the main polymer chain. In contrast to syndiotactic polymers, isotactic polymers generally are characterized as having the methyl groups on the repeating units with identical sequence configurations as contrasted with the alternating configurations of syndiotactic polymers. Such structures may be described by conventional and well-known graphical presentations, such as Fischer projection and corresponding NMR pentad sequences as disclosed, for example, in U.S. Pat. Nos. 5,334,677 to Razavi et al and 4,522,982 to Ewen. While isotacticity and syndiotacticity are useful in defining these two broad types of crystalline polymer configurations, alternatives of both are known in the prior art. For example, so-called stereoblock polymers, such as disclosed in the aforementioned '982 patent, may be involved. Also a specialized form of isotactic polypropylene in which alternative polymer units achieve a random asymmetricity can be formed as stereoblock polymers which can be formed, for example, of alternating isotactic blocks. Various monomers which can be stereospecifically propagated include the ethylenically unsaturated monomers such as C.sub.3 +alpha olefins, such as propylene and 1-butene; dienes, such as 1,3-butadiene; substituted vinyl compounds, such as vinyl chloride or vinyl aromatic compounds, e.g. styrene; and vinyl ethers, such as alkyl vinyl ethers, e.g. isobutylvinyl ether or even arylvinyl ethers. As indicated above, the most significant application of stereospecific polymerization is in the production of isotactic or syndiotactic polypropylene.
The catalyst systems useful in the formation of isotactic polyolefins include the racemic bis-indenyl compounds of the type disclosed in U.S. Pat. No. 4,794,096 to Ewen. Those useful in the propagation of syndiotactic polypropylene and like syndiotactic polymers include stereorigid metallocenes having different substituted cyclopentadienyl groups, e.g. bridged cyclopentadienyl fluorenyl ligands, as disclosed, for example, in U.S. Pat. No. 5,334,677 to Razavi et al and U.S. Pat. No. 5,155,080 to Elder et al. A variation of such cyclopentadienyl fluorenyl ligand structures, which are substituted so as to produce a lack of bilateral symmetry, are disclosed in U.S. Pat. No. 5,036,034 to Ewen to produce hemi-isotactic polypropylene.
While the foregoing structures can be generally characterized as having fluorenyl-cyclopentadienyl ligand structures, another class of compounds having bis-fluorenyl ligand structures are known in the art as useful in producing crystalline polymer structures involving isotactic or syndiotactic polymer chains. Thus, U.S. Pat. No. 5,401,817 to Palackal et al discloses silicon bridged bis-fluorenyl compounds in which one fluorenyl group is unsubstituted or symmetrically substituted and the other is a different fluorenyl radical which is unsubstituted or symmetrically substituted. Alternative ligand configurations are disclosed in Palackal et al in which only one fluorenyl group is involved, but at least one of the groups in the ligand structure, characterized in Palackal as being a silyl bridged sandwiched metallocene, must be a fluorenyl group. The substituents of the silyl bridge may be identical or different C.sub.1 -C.sub.20 organo radicals, such as fluoro alkyl or aryl groups, alkyl groups, aryl groups, or alkylaryl groups. Characteristic of the symmetrical ligand structures disclosed in Palackal are 9-(2,7-di-t-butylfluorenyl)-9'-fluorenyldiphenylsilane and the corresponding 3-6 alkyl derivative. As is normally the case with such metallocene catalysts, the silyl bridged metallocenes of Palackal are employed in conjunction with a cocatalyst such as an alumoxane which may be used alone or in conjunction with a trialkyl aluminum.
Other examples of fluorenyl-based metallocenes are disclosed in U.S. Pat. No. 5,451,649 to Zenk et al. In Zenk et al, both bridged and unbridged metallocenes are disclosed. Particularly preferred in Zenk et al are bridged metallocenes containing at least one symmetrically substituted fluorenyl radical. Suitable symmetrically substituted fluorenyl groups include 2,7-di-alklyfluorenyls, the corresponding 3,6 and 1,8 derivatives and 2,3:6,6 and 3,4:5,6 dibenzofluorenyls. As disclosed in Zenk, the alkyl derivatives may range from 1 to 20 carbon atoms with 1 to 6 being preferred. Aryl substituents may range from 6 to 20 carbon atoms with 6 to 10 carbon atoms being preferred for unsubstituted aryls and 7 to 10 carbon atoms being preferred for aryl alkyl substituents. Various alpha olefins, disclosed as being suitable for polymerization by the Zenk et al catalysts, include ethylene, propylene, 1-pentene, 1-heptene, 1-octene, 1-decene and various alkyl and dialkyl substituted derivatives of such alpha olefins.
Similarly bridged and unbridged metallocenes derived from fluorenyl ligands are disclosed in Canadian Patent No. 2,067,524 to Alt et al. Here such bridged or unbridged compounds are said to be useful catalysts in the production of polymers such as crystalline and a noncrystalline polypropylene. For example, Alt et al discloses the use of 1,2-di(2-tert butyl fluorenyl)ethane zirconium dichloride to produce crystalline polypropylene. Various techniques are disclosed in Alt for the production of various alkyl substituted fluorenes from the correspondening carboxylic acid fluorenes. For example, as disclosed in Alt, 1-carboxylic acid fluorenone can be converted to 1-methylfluorene by a reaction route involving the intermediate 1-hydroxymethyl fluorenone. Similar reaction routes can be used to produce 1-isopropyl fluorene and 2-tert-butyl fluorene as well as 4-fluorenyl derivatives such as 4-methyl fluorene and 4-tert-butyl fluorene.
Yet another disclosure of bis-fluorenyl metallocene catalysts is found in U.S. Pat. No. 5,459,117 to Ewen. Ewen discloses various metallocenes including bis-fluorenyl metallocenes which are said to be useful in the catalysis of isotactic or syndiotactic polypropylene. The stereorigid metallocenes of Ewen are disclosed there as being "doubly-conformationally locked" with substituted metallocenes characterized as having C.sub.2 or C.sub.s symmetry or pseudo-C.sub.2 or C.sub.s symmetry. Such metallocene ligands are characterized as having bis(4,5-di-substituted fluorenyl) ligands with C.sub.2 or pseudo-C.sub.2 symmetry resulting in an isospecific catalyst and with a C.sub.s or pseudo-C.sub.s symmetry resulting in a syndiospecific catalyst. The catalyst ligand structures have dissimilar substituents at the 4 and 5 positions of the fluorenyl groups with C.sub.s (or pseudo-C.sub.s) symmetry resulting when the 4,5 substituents of one fluorenyl group or sterically smaller than the corresponding substituents of the other fluorenyl group, and C.sub.2 or pseudo-C.sub.2 symmetry resulting when one fluorenyl group has substituents which are smaller and larger than substituents on the other fluorenyl group. Suitable substituent groups at the 4 and 5 positions on the fluorenyl groups are said to include C.sub.1 -C.sub.20 hydrocarbyl groups including heterosubstituted groups such as silicon, phosphorus, boron and nitrogen derivatives which can be linear or branch chain, as well as various C.sub.3 -C.sub.20 cyclohydrocarbyl groups and aryl or alkylaryl groups. The bridge structure in Ewen is described broadly as a C.sub.1 -C.sub.20 alkyl and bridges incorporating silicon, germanium, phosphorous, boron and aluminum which may be substituted with alkyl or aryl groups.
Yet another group of bis-fluorenyl metallocenes characterized as having C.sub.2v or C.sub.2 symmetry and their use as alpha olefin polymerization catalysts are disclosed in Chen et al, Macromolecules, "C.sub.2v and C.sub.2 -Symmetric ansa-Bis(fluorenyl)zirconocene Catalysts: Synthesis and .alpha.-Olefin Polymerization Catalysis," Vol. 28, No. 16, Jul. 31, 1995. Specifically disclosed in Chen et al are rac- and meso-ethylenebis9-(l-methyl)fluorenyl!zirconium dichloride. In Chen et al a diastereomeric mixture of the rac- and meso-diastereomers was employed in a polymerization reaction to produce isotactic polypropylene having a relatively high mmmm pentad distribution. A somewhat similar disclosure is found in European Patent Application No. 628,565 to Patsidis et al. Here, metallocene structures characterized as bis-fluorenyl bridged sandwich-bonded metallocenes, having 1-methyl substitution as in the Chen et al article, were disclosed as useful with a mixture of racemic and meso isomers to produce isotactic polypropylene.